1. Field of the Invention
The present invention relates to a thermistor-type temperature sensor for measuring exhaust gas temperature of an internal combustion engine or the like.
2. Description of the Related Art
Among the wide variety of temperature sensors presently available, a typical one is a thermistor-type sensor in which temperature-resistance characteristics of a thermistor element are used.
To measure temperature by the thermistor-type sensor, the thermistor element on the tip of an insulation tube is put close to the subject to be measured. The thermistor element and the insulation tube are encapsulated in a metal tube to shield and protect the element.
Japanese unexamined patent publication No. 62-278421 teaches one structure in which heat-resistant cement fixes the thermistor element within the insulation tube within the metal tube is used. For example, as shown in FIG. 4 illustrating a sensor 9, a thermo unit 90 which has a thermistor element 91 stuck out from the end of insulation tubes 921 and 922 is fixed to a metal tube 93 by heat-resistant cement 94. Before the thermo unit 90 is inserted into the metal tube 93, electrical lead wires 95 of the thermistor element 94 are connected with output signal lead wires 96 at connection portion 951. A threaded metal fixture 97 is fixed to the metal tube 93 so that the sensor 9 may be screw-threaded into an exhaust pipe.
However, the above structure has the following drawbacks. In the production process, particularly in the process of filling the cement 94 into the closed bottom of the metal tube 93, fine volume control of the cement becomes very important because it determines depth or axial length of the cement 94 filled in the metal tube 93. It also determines the quality of the sensor as explained hereunder.
That is, if the volume of the cement 94 filled into the metal tube 93 is less than a specified appropriated volume, it makes the filled length shorter than that specified and results in, for example, the disconnection of the output signal lead wires 96 and the electrical lead wires 95 by the lack of anti-vibration strength due to the insufficient holding strength of the thermo unit 90.
On the other hand, if the volume of the cement 94 is much more than that specified, it makes the length longer than specified and the cement 94 covers the connection portion 951. This results in the deterioration of electrical insulation between the output signal lead wires 96 and the electrical lead wires 95, an increase of electrical leakage current therebetween, and an increase in the measuring error of the temperature due to the growth of an electric parallel circuit. The connection portion 951 is practically covered by insulation material such as a teflon tube. However, the cement 94 may intrude into the teflon tube and water which will be contained in the cement 94 even after drying causes a reduction in the electrical insulation resistance to a value less than several kilo-ohms.
The gap G between the inner diameter of the metal tube 93 and the outer diameter of the thermo unit 90 is specified as small as possible to hold the thermo unit 90 and the metal tube 93 coaxially parallel. As a conventional solution, it can be thought to sufficiently extend the axial length of the metal tube 93 and the thermo unit 90 to make a space for a cement filling. However, this sensor structure does not meet the practical requirement for compactness.